1. Field of the Invention
The present invention relates to a process for making an economical light waveguide element mounted on a mount board along with an optical fiber and photoelectric components and enabling precise optical axis alignment.
2. Description of the Related Art
Japanese patent application Kokai No. 10-133069 discloses a method of mounting a light waveguide element, by which the light waveguide element is mounted on the same silicon substrate along with the optical fiber and photoelectric components to which the light waveguide element is connected. It is mounted such that the upper surface of the light waveguide element layer faces the standard surface of the silicon substrate so that the thickness of the clad layer of the light waveguide element is adjusted to match the optical axis height from the silicon substrate of the optical fiber and photoelectric components. That is, the optical axis adjustment is made by only the mounting precision of each optical component to provide an economic light transmission module.
The thickness of the upper clad layer for the light waveguide element must be 15 μm or more for light propagation characteristics. The general optical axis height of the semiconductor laser and photodiodes is approx. 10 μm so that it is necessary to make the upper clad layer approx. 10 μm thick. Consequently, the upper clad layer has a T-shaped section that is thick above the core and thin at the height-adjusting sections. In order to provide such a T-shaped layer, a conventional technique comprises forming an upper clad layer having a thickness of approx. 15 μm and etching off, by reactive ion etching or the like, only the height-adjusting sections of the upper clad layer.
In such a conventional method, however, the etching rate depends on the etching depth and the reaction product adhered to the chamber so that it is difficult to make the etching stable. Consequently, it has been necessary to take height measurements and repeat the etching with correction. In addition, if overetching is made, it is necessary to deposit an additional upper clad, increasing the number of process steps.
Moreover, variations in the etching rate in the chamber or the wafer surface make it difficult to provide uniform etching between wafers or within the wafer surface, resulting in the reduced yield. Thus conventional process has low productivity and fails to produce the light waveguide element economically.